In the area of electronic distance measurement, various principles and methods are known. One approach consists in emitting frequency-modulated electromagnetic radiation, such as, for example light, to the target to be surveyed and then receiving one or more echoes from back-scattering objects, ideally exclusively from the target to be surveyed, it being possible for the target to be surveyed to have both a reflective—for example retroreflectors—and a diffuse back-scattering characteristic.
After reception, the optionally superposed echo signal is superposed with a mixed signal and the signal frequency to be analyzed is reduced thereby so that less outlay is required with respect to the apparatus. The mixing can be effected either as a homodyne method with the signal sent or as a heterodyne method with a periodic, in particular harmonic, signal of known period. Thus, the methods differ in that mixing is effected with the transmitted signal itself or with a harmonic signal having its own frequency. The mixing serves for transforming the received signal to lower frequencies and for amplifying said signal. Thereafter, the transit times and hence—in the case of a known propagation velocity of the radiation used—the distances to the targets to be surveyed are determined from the resulting signal. In a heterodyne interferometer arrangement, a tuneable laser light source is used for the absolute distance measurement. In the embodiment which is simplest in principle, the tuning of the optical frequency of the laser source is effected linearly. The received signal is superposed with a second signal which is derived from the emitted light signal. The resulting beat frequency of the heterodyne mixed product, the interferogram, is a measure of the distance to the target object. The apparatuses used for implementing these methods usually utilize a signal generator as a chirp generator, which impresses a signal on a modulatable radiation source. In the optical range, lasers which can be chirped by modulation of the external (for example Bragg grating) or internal cavity (for example Distributed Feedback (DFB) or Distributed Bragg Reflector (DBR)) are generally used as radiation sources. In the optical range, transmitting and receiving optical systems to which a detector or quadrature detector for heterodyne mixing, A/D convertor and digital signal processor are connected down-circuit are used for emission and for reception.
An example of an optical, coherent FMCW distance-measuring method is described in U.S. Pat. No. 4,830,486, this method having an accuracy in the region of phase-measuring methods in combination with a short measuring time. A chirp generator produces a linear frequency-modulated signal which is divided into a measuring signal and local oscillator signal, the two signals being superposed in a receiver.
The change in the wavelength of the emitted light signal represents the scale of the measurement. This is generally not known and therefore has to be determined in an additional measurement. For this purpose, in the prior art, for example, a part of the emitted light is passed via a reference interferometer having a defined reference length. The change in the wavelength of the emitted light signal as a function of time can be inferred from the resulting beat product on the basis of the known reference length. If the reference length is not known or is unstable, for example owing to temperature influences, it can be determined via an additional calibration unit, for example a gas cell or a Fabry-Perot element.
EP 1 696 201 discloses such a distance-measuring method comprising emission of frequency-modulated electromagnetic radiation to at least one target to be surveyed and subsequent reception with heterodyne mixing of the radiation scattered back from the target, the radiation being passed in parallel over an interferometric reference length.
While a stationary target has a defined distance invariable as a function of time, moving or vibrating targets present some problems. A constant movement of the target leads, during tuning, to opposite Doppler shifts for the different directions of the frequency ramp. Thus, a movement leads, for example, to a positive Doppler shift on passing through an ascending frequency ramp, whereas a negative Doppler shift is produced in this case On passing through the descending ramp. By using successive ascending and descending ramps, this effect can be compensated.
However, the use of ramps following one another as a function of time, i.e., or opposite chirps of the laser radiation, also reduces the useable measuring rate by a factor of two, for example from 1 kHz to 500 Hz, i.e. to half. Moreover, this approach is based on the fact that there is a constant target velocity during the time taken for passing through the two ramps. Accelerations of the target during the measuring process or vibrations cause errors in the measured distance.
In order to eliminate this problem, U.S. Pat. No. 7,139,446 proposes using two simultaneous and opposite frequency ramps, i.e. emitting radiation having two radiation components with opposite chirp, which also avoids a reduction of the measuring rate. In order to be able to separate these radiation components with respect to measurement, the emission and detection thereof is effected with different polarization. By means of this approach, accelerations can be detected and vibrations eliminated. The separation of the two radiation components by the different polarization does however require that preservation of polarization is ensured within the setup. A setup using fibre optics therefore requires polarization-preserving fibres and is susceptible to errors in the mutual orientation of the fibres relative to one another or the design of the connections. Moreover, this approach is based on the fact that the target too has polarization-preserving properties, i.e. and the target results in no influences at all on polarization. For producing the two radiation components, two laser sources coupled in in orthogonal polarizations and two detectors oriented in orthogonal polarizations are used.
However, the use of polarized light has disadvantages in measurements to metallic surfaces. Metallic surfaces having a microroughness, as is usual in the case of technical surfaces, lead to depolarization in the case of obliquely incident light. This was investigated in detail in the prior art, both experimentally, cf. for example K. A. O'Donnell and E. R. Mendez, “Experimental study of scattering from characterized random surfaces”, J. Opt. Soc. Am. A/Vol 4, No. 7, July 1987, pages 1194-1205, or Gareth D. Lewis et al., “Backscatter linear and circular polarization analysis of roughened aluminum”, Applied Optics, Vol. 37, No. 25, September 1998, pages 5985-5992 and theoretically in simulations, cf. for example E. R. Mendez et al., “Statistics of the polarization properties of one-dimensional randomly rough surfaces”, J. Opt. Soc. Am. A, Vol. 12, No. 11, November 1995, pages 2507-2516, and G. Soriano and M. Saillard, “Scattering of electromagnetic waves from two-dimensional rough surfaces with an impedance approximation”, J. Opt. Soc. Am. A, Vol. 18, No. 1, January 2001, pages 124-133, so that a substantial limitation of the usability of the method described in U.S. Pat. No. 7,139,446, results.
The usability of interferometric distance-measuring methods of the prior art with high measuring rates, which are also suitable for surveying vibrating or moving targets, is therefore not possible or possible only under restricting conditions for metallic surfaces.